It is well recognized that rapid transfusion of multiple units of refrigerated (standardly, at 4 degrees C.) blood (whole blood, packed red blood cells, or fresh frozen plasma) without rewarming such blood to body temperature can be hazardous, causing hypothermia, cardiac arrhythmias, and coagulopathy. Cold whole blood and packed red blood cells are more viscous than warmed products, and do not achieve the same rates of flow, which decreases the rate of administration. Fresh frozen plasma, which is stored frozen, requires 20 to 30 minutes for thawing and rewarming by currently-utilized techniques.
A number of techniques have been attempted for blood rewarming, including direct heating, water-bath/coil heating, and microwave heating. Of these, the only successful method to date has been the water bath technique in which blood is passed through coiled tubing immersed in a temperature-controlled water bath. The blood is carried through the tubing by gravity or by a mechanical pump. While that technique is reliable to the extent that it avoids overheating of the blood, it carries a number of disadvantages. The apparatus required for providing rapid transfusion of multiple units is expensive in terms of both initial costs and operating costs. The equipment is unwieldy, inconvenient to use, and only marginally portable since the entire water bath must be transportable. Because of its cumbersome nature, such equipment is often unmanageable in emergency departments and operating rooms. Finally, larger blood warming equipment generally requires that the blood utilized first be "donated" to the equipment so that if multiple units are not transfused they become wasted.
For these reasons, and because conventional water-bath/coil/pump systems require costly priming and considerable set-up time, efforts began in the mid-1960's to develop microwave warmers for rapid heating of blood in plastic bags then (and currently) in widespread use worldwide. While initially met with enthusiasm, these methods have been denounced and abandoned, largely because of problems of differential excessive heating which caused red cells to hemolyze, rendering them useless for transfusion. Despite considerable developmental effort, the problems of differential heating could not be solved by mechanical agitation or by rotating cylinders used to effect constant admixture. Segments of narrow diameter tubing, which are integral parts of blood storage bags, frequently overheated and even exploded. Quality control to avoid overheating was considered impossible because of variability in the volume of blood units and effective temperature/time surveillance. Microwave rewarming of fresh frozen plasma (in contrast to that of whole blood and packed red cells) has been achieved successfully, but the technique is complicated, involving steps of first softening the bag and its contents under warm running water, followed by sealing and removal of all tubing to avoid bursting. The bag is then dried and placed in a plastic overbag, protecting all metal clips and remaining narrow tubulature. Microwave exposure is applied in short (10 second) increments under constant observation until the thaw is completed. At the conclusion, the plasma still remains considerably below body temperature. Because of the complexity of this technique, most blood banks continue to utilize the water bath method for thawing plasma.
Investigators have demonstrated that microwave energy does not appear to be intrinsically harmful to blood in vitro or in vivo as long as the temperature does not reach the point at which hemolysis occurs. Microwave energy does not appreciably affect the survival or activity of red cells, nor does it render the coagulation factors in plasma less effective, as determined by chemistry/ electrolyte determinations, radioactive chromium tags, serum haptoglobin and adenosine triphosphate measurements. The major difficulty that has led to the abandonment of microwave energy utilization for rewarming banked blood has been the conversion of microwave energy to heat without overheating the blood products.
An important aspect of this invention therefore lies in the discovery that it is indeed possible to thaw and/or warm banked blood using microwave energy in a controlled method without inflicting damage to the constituents of that blood. Compared with conventional water-bath/coil/pump systems, the method and apparatus of this invention involve zero set-up time, 5 to 6 minute warming time for refrigerated whole blood or packed red blood cells and 7 to 8 minute warming time for FFP, a high degree of mobility of the apparatus (since no water connections are needed), minimal storage and operating space, relatively low capital investment, and negligible operating expense. The method and apparatus of this invention eliminate the complications previously encountered with microwave heating of blood, with tests revealing that the warmed product is free from hemolysis occasioned by localized overheating and retains all of the properties of fresh stored blood.
The term "blood" as used herein is intended to include not only refrigerated whole blood but also the separated and/or treated components of whole blood. Accordingly, the term includes both packed red blood cells (PRBC) and fresh frozen plasma (FFP), all of which are routinely processed and stored in blood banking operations. Particularly in the United States, but in other countries as well, such blood is placed in plastic bags or pouches for storage and subsequent infusion, all well-understood in the health care field.
Briefly, the apparatus and method involve supporting a blood bag in a microwave-transmissible housing containing a body of water and then immobilizing the blood bag so that water completely surrounds the stationary bag. The water and bag are subjected to microwave irradiation until the temperature of the water adjacent the bag reaches a predetermined temperature. While certain portions of the bag may be shielded against direct exposure to such irradiation, the primary factor responsible for the generally uniform heating of the blood, without localized overheating or "hot spots" that might result in hemolysis, is the presence of the surrounding body of water.
A body of water is believed to function as a heat sink to draw excess heat away from the bag and thereby avoid a large temperature gradient between the core liquid and the liquid adjacent to the inner surface of the bag. The water absorbs both microwave energy directly from the source and heat energy from the bag, so that the blood temperature never exceeds that of the water.
In a preferred embodiment, means are provided for circulating the water surrounding the blood bag; however, it is believed that such circulation, while desirable, may not be essential. A temperature detector or probe with its sensing tip alongside the immobilized blood bag, at about the level of the horizontal midplane thereof, detects the temperature of the water and automatically interrupts the heating operation when a selected blood temperature (generally within the range of 94 to 98 degrees F.) is reached.
The blood bag is supported horizontally within the immersion chamber, and the immobilizing means takes the form of a horizontal platen formed of rigid microwave-transmissible material. The platen bears against the upwardly-facing side wall of the horizontally-oriented bag, and retaining springs urge the platen downwardly to hold the blood bag in place. That area of the platen in direct contact with the bag (which is also in generally vertical alignment with the microwave-generating source directly thereabove), as well as selected portions of the platen overlying the outlet fittings of the bag, may be provided with an opaque layer or coating capable of blocking microwave transmission. The effect is to provide shielding for those portions of the bag that are more directly exposed to microwaves and might be heated more quickly than other portions of the bag and its contents.
The apparatus, in its simplest form, may comprise a unit capable of being placed within a conventional microwave oven. Alternatively, the invention contemplates that the apparatus may be part of a microwave oven or heating appliance specifically designed and constructed for rewarming blood.